Such interconnections commonly are by holes through the board (called vias) providing a path which is plated or filled with a conductive material. This may be unsuitable, however, when the circuit board contains an inner metal layer since such a metal layer must be insulated at each via. Such insulation adds expense and creates a potential at each via of minute holes and other imperfections resulting in failures of the circuit board. Since such insulation normally is achieved by plating from a wet bath, insulating such vias is particularly undesirable where the populated circuit board otherwise can be fabricated in a largely dry process since wet baths require steps such as cleaning before immersion, immersion, cleaning after immersion, and drying so as not to contaminate the immersion chemicals during immersion and so as to neutralize or eliminate the residue.
Self-supporting conductors connecting electrical elements across the board are not known to be used commonly, but that is described in U.S. Pat. No. 3,977,074 to Furnival. That connection is by forcing a connecting metal tab through a via and bonding it at the sides of the via, or by bending the wire to be flat on the opposite side of the board, where the bonding is said to be by soldering, resistance welding, or ultrasonic welding, for example.
This invention has particular utility for use with a circuit board having a metal foundation layer, such as copper or aluminum. The purpose of metal as the foundation layer is to dissipate heat rapidly by the metal readily conducting it. The heat is therefore dispersed by the metal layer and thereby readily transferred to the surroundings. With metal as a foundation, circuit elements can be applied with minimal wet chemistry.
Specifically, a thin insulating layer of polyimide can have a copper circuit pattern plated or painted on it, all by conventional techniques. Alternatively, a metal sheet is applied by adhesive on the polyimide layer and the circuit pattern is created by wet etching, as is conventional. Nickel topped with gold is then applied by standard wet plating where contacts are to be made. This lamination is then moved bodily on to the copper foundation sheet, where it is cemented by an adhesive with the circuit pattern outward. Discrete circuit elements are then applied to the circuit pattern by surface mount techniques.
The foundation layer forms no part of the electrical circuitry, although commonly one or more connections are provided to it for connection to ground potential, so that the copper foundation serves as both a heat exchanger and a ground plane. Heat dissipation becomes more important with increasing circuit densities now being realized.